2016 day15 done

3 files changed, 44 insertions, 3 deletions

diff --git a/src/2016/day15/README.md b/src/2016/day15/README.md
index 4de57e7..c3d6cf4 100644
--- a/src/2016/day15/README.md
+++ b/src/2016/day15/README.md
@@ -23,3 +23,11 @@ If, however, you wait until time=5 to push the button, then when the capsule rea
 
 However, your situation has more than two discs; you've noted their positions in your puzzle input. What is the first time you can press the button to get a capsule?
 
+--- Part Two ---
+
+After getting the first capsule (it contained a star! what great fortune!), the machine detects your success and begins to rearrange itself.
+
+When it's done, the discs are back in their original configuration as if it were time=0 again, but a new disc with 11 positions and starting at position 0 has appeared exactly one second below the previously-bottom disc.
+
+With this new disc, and counting again starting from time=0 with the configuration in your puzzle input, what is the first time you can press the button to get another capsule?
+
diff --git a/src/2016/day15/aoc.cpp b/src/2016/day15/aoc.cpp
index e188291..318b345 100644
--- a/src/2016/day15/aoc.cpp
+++ b/src/2016/day15/aoc.cpp
@@ -2,5 +2,38 @@
 
 namespace aoc2016 {
 
-std::pair<int64_t, int64_t> day15(line_view) { return {0, 0}; }
+size_t position_at(int t, int slot, size_t p) { return t > 0 ? (t % slot + p) % slot : p; }
+
+struct disc {
+  int slots;
+  size_t p;
+};
+
+static bool same(size_t* ps, int x) {
+  for (int i = 0; i < x; i++) {
+    if (ps[i] != ps[x]) {
+      return false;
+    }
+  }
+  return true;
+}
+
+int first_catch(disc ds[7], int x) {
+  int t{0};
+  while (true) {
+    size_t ps[7] = {0, 0, 0, 0, 0, 0, 0};
+    for (size_t i = 0; i < 7; i++) {
+      ps[i] = position_at(t + i + 1, ds[i].slots, ds[i].p);
+    }
+    if (same(ps, x)) {
+      return t;
+    }
+    t++;
+  }
+}
+
+std::pair<int64_t, int64_t> day15(line_view) {
+  disc ds[7] = {{17, 1}, {7, 0}, {19, 2}, {5, 0}, {3, 0}, {13, 5}, {11, 0}};
+  return {first_catch(ds, 5), first_catch(ds, 6)};
+}
 } // namespace aoc2016
diff --git a/test/test_2016.cpp b/test/test_2016.cpp
index 19e6ad0..658acce 100644
--- a/test/test_2016.cpp
+++ b/test/test_2016.cpp
@@ -141,8 +141,8 @@ TEST_CASE("One-Time Pad", "[2016]") {
 TEST_CASE("Timing is Everything", "[2016]") {
   line_view lv = load_file("../src/2016/day15/input");
   auto p = aoc2016::day15(lv);
-  REQUIRE(0 == p.first);
-  REQUIRE(0 == p.second);
+  REQUIRE(317371 == p.first);
+  REQUIRE(2080951 == p.second);
 }